Wind turbines are devices that convert mechanical energy to electrical energy. A typical wind turbine includes a nacelle mounted on a tower housing a drive train for transmitting the rotation of a rotor to an electric generator.
The efficiency of a wind turbine depends on many factors. One of them is the orientation of the rotor blades with respect to the direction of the air stream, which is usually controlled by a pitch system that allows adjusting the pitch angle of the rotor blades for maintaining the rotor's speed at a constant value or within a given range. Otherwise, specially at high wind speeds, the load of the rotor will exceed the limits set by the wind turbine's structural strength.
There are two basic methods for controlling the power of a wind turbine changing the pitch angle of the rotor blades: the “pitch” control method and the “stall” control method.
In the “pitch” control method the rotor blade's pitch angle is changed to a smaller angle of attack in order to reduce power capture and to a greater angle of attack to increase the power capture. This method allows a sensitive and stable control of the aerodynamic power capture and rotor speed.
In the “stall” control method the rotor blade's pitch angle is changed to a greater angle of attack to the point where the flow separates at the rotor blade's surface, thus limiting the aerodynamic power capture.
The pitch regulated wind turbines can also use the pitch system to reduce the dynamic loads, either by cyclic pitch or by individual blade pitch. However, for large wind turbine blades it can be difficult to control the blade loading as the blade loading can vary over the blade length. As the rotor size is increasing, the pitching of the blades not necessarily provides an optimized loading along the whole blade because nor only wind shear, yaw errors and gust will affect the flow on the blade, but different gusts can hit the blade simultaneously or complex wind shear profiles with negative wind shear can occur.
In addition to the use of the pitch system there are known in the prior art some proposals in the prior art for optimizing the blade loads.
One known proposal is the use of small control surfaces such as Gurney flaps attached to the trailing edge for optimizing the blade loads. One disadvantage of Gurney flaps is the increase in aerodynamic noise from the free ends of the Gurney flaps and from the gaps in the blade where the Gurney flap is positioned.
Another known proposals are addressed to control the aerodynamic forces along the rotor blades by a continuous variation of the airfoil geometry in the leading edge region and trailing edge region along part of or along the whole blade span.
One of these proposals, disclosed in WO 2004/088130, relates to a design concept by which the power, loads and/or stability of a wind turbine may be controlled by a fast variation of the geometry of the blades using active geometry control (e.g. smart materials or by embedded mechanical actuators), or using passive geometry control (e.g. changes arising from loading and/or deformation of the blade) or by a combination of the two methods. In one preferred embodiment piezoelectric plates are to built in the trailing edge over part of the blade for modifying its geometry in order to reduce the blade loads. One disadvantage of the piezoelectric plates are the electrical cables that are necessary to bring power to them. These cables are woundable to electrical lightning and can easily be damaged in case of a lightning strike.
Another proposal, disclosed in U.S. Pat. No. 6,769,873, relates to a dynamically reconfigurable wind turbine blade assembly including a plurality of reconfigurable blades mounted on a hub, an actuator fixed to each of the blades and adapted to effect the reconfiguration thereof, and an actuator power regulator for regulating electrical power supplied to the actuators.
None of these proposals produces fully satisfactory results, therefore a continuing need exists for wind turbines having rotor blades with means for reducing the blade loads.